Visual monitoring systems, particularly those developed for use in industrial environments where conditions for observation of objects of interest are often detrimental to reliable viewing, generally require means to maintain a clear view of the objects of interest. These environments commonly have ambient conditions that result in the monitoring equipment being coated in short order in layers of opaque material that are by-products of the industrial process under observation. For example, in the paper making industry, stray paper fibres that are an inevitable part of the manufacturing process tend to be deposited on, and build up to cover all stationary equipment adjacent to the processing line. The ambient conditions are also inappropriate for reliable operation of electronic monitoring equipment such as video or still cameras. The environments can include high temperatures, caustic or corrosive atmospheres, condensing liquid and directed sprays, and mechanical contaminants and debris that will individually or in combination cause the camera equipment to malfunction or to fail to provide the required view.
Visual monitoring systems will typically protect the image acquisition equipment or camera in an enclosure to provide means to control the environment in which the camera operates. This enclosure is substantially sealed from the external environment and is equipped with a covered view port through which the camera can capture images of the external environment. Generally, the enclosure will have a manual or automatic system to maintain a clear view port for the camera. Many such systems rely on recessed view ports and/or a sheet of air that is operated intermittently or continuously to prevent debris or other contaminants from blocking the view port. For example, U.S. Pat. No. 4,969,035 to Dawson discloses the use of a gas jet to prevent blocking of a pin hole aperture adjacent a camera window with debris to permit viewing of the interior of a furnace or similar high temperature environment. Published US Patent Application No. 20030210906 to Peterson et. al. teaches the use of intermittent bursts of air to clean a cover positioned adjacent a camera protected with an enclosure. Published US Patent Application No. 20050276599 to Kajino et al. discloses the use of air streams directed past the lens of a camera sealed in a housing to prevent condensation built up from blocking the view of the camera.
In another approach, U.S. Pat. No. 5,394,208 to Campbell discloses an enclosure for a camera that relies on creation of a pressurized enclosure interior and formation of a vortex flow through a camera view port in the enclosure to prevent blocking of the view port.
U.S. Pat. No. 6,362,889 to Mustonen shows the use of a continuous air stream that flows through a camera housing and across the camera lens to prevent accumulation of blocking debris.